The present invention relates to a brake boosting system for providing intensified braking force by directly boosting master cylinder pressure.
Conventionally, a brake boosting system used in an automobile is provided with a booster, which boosts input to a predetermined magnitude by using hydraulic pressure such as negative pressure or fluid pressure and a master cylinder (hereinafter, sometimes referred to as "MCY") is actuated by the output of the booster, thereby obtaining large braking force from small leg power on the brake pedal.
Conventionally used in a vacuum brake boosting system provided with a vacuum booster is negative pressure produced in an engine. Recently, however, fuel consumption reduction of an engine is desired, whereas lower negative pressure is produced in the engine. As a result of this, output of a vacuum booster is also reduced. One of measures to cope with this reduction in the output is enlargement of the vacuum booster. However, this enlargement makes it quite difficult to install the vacuum booster in a small engine room.
On the other hand, a fluid pressure brake boosting system provided with a fluid pressure booster has a problem that parts such as a pump and an accumulator are required, thereby increasing the cost.
In such brake boosting systems, it is desired that, in case of emergency braking, the boosting device quickly produces braking force larger than that in case of normal braking.
among inexpert drivers, there must be a person who can not pedal a brake pedal fully so as not to develop large braking force. In this case, it is desired to provide braking assist for ensuring that large braking force is obtained even by such an inexpert driver.
In the conventional brake boosting systems, however, since the servo ratio of the servo control is constant during the output of the braking force, not only braking force larger than that for normal braking can nor be quickly produced for emergency braking, but also no braking assist for ensuring that large braking force is obtained even by the inexpert driver is provided. Therefore, it is difficult to meet the aforementioned demands.